Golf ball

ABSTRACT

A golf ball  2  has a spherical core  4 , a mid layer  6  situated on the external side of the core  4 , and a cover  8  situated on the external side of the mid layer  6 . The core  4  has a spherical center  10  and an envelope layer  12  situated on the external side of the center  10 . At all points P included in a zone away from the central point of the core  4  at a distance of 1 mm or greater and 15 mm or less, the following mathematical expression is satisfied. 
         H 2− H 1≦5
 
     In the above mathematical expression, H 1  represents the JIS-C hardness at a point P 1  that is located inside the point P along the radial direction and away from the point P at a distance of 1 mm, and H 2  represents the JIS-C hardness at a point P 2  that is located outside the point P along the radial direction and away from the point P at a distance of 1 mm.

This application claims priority on Patent Application No. 2009-297651filed in JAPAN on Dec. 28, 2009. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. More particularly, thepresent invention relates to multi-piece golf balls having a core, a midlayer and a cover.

2. Description of the Related Art

Top requirement for golf balls by golf players is their flightperformances. The golf players place great importance on flightperformances achieved upon shots with a driver, a long iron and a middleiron. The flight performances correlate with resilience performances ofthe golf ball. Hitting of a golf ball that is excellent in resilienceperformance leads to a high-speed flight, whereby a great flightdistance is attained.

For attaining a great flight distance, an appropriate trajectory heightis required. The trajectory height varies depending on the spin rate andlaunch angle. Golf balls which achieve a high trajectory due to a highspin rate are accompanied by insufficient flight distance. Golf ballswhich achieve a high trajectory due to a great launch angle can attain agreat flight distance. By employing a core having anouter-hard/inner-soft structure, a low spin rate and a great launchangle can be both achieved.

Golf players place great importance also on spin performances of golfballs. A great back spin rate results in small run. For golf players,golf balls which are liable to be spun backwards are apt to be renderedto stop at a target position. Great side spin rate results in easilycurved trajectory of the golf ball. For golf players, golf balls whichare liable to be spun sidewise are apt to allow their trajectory tocurve intentionally. The golf balls that can be easily spun areexcellent in control performances. High-level golf players particularlyplace great importance on control performances upon shots with a shortiron.

In light of achievement of various performances, golf balls having amultilayer structure have been proposed. Japanese Unexamined PatentApplication, Publication No. H10-328326 (equivalent to U.S. Pat. No.6,468,169) discloses a golf ball having an inner sphere, an enclosurelayer, an inner cover and an outer cover. Japanese Unexamined PatentApplication, Publication No. 2001-17575 (equivalent to U.S. Pat. No.6,271,296) discloses a golf ball having a core, an envelope layer, a midlayer and a cover. Japanese Unexamined Patent Application, PublicationNo. 2002-272880 (equivalent to US 2001/0024982) discloses a golf ballhaving a core and a cover. This core is composed of a center and anouter core layer. The cover is composed of an inner cover layer and anouter cover layer. Japanese Unexamined Patent Application, PublicationNo. 2003-205052 (equivalent to US 2003/0166422) discloses a golf ballhaving a center, a mid layer and a cover. Japanese Unexamined PatentApplication, Publication No. 2004-130072 (equivalent to US 2004/0029648)discloses a golf ball having a core and a cover. This core has athree-layer structure.

When a core having an outer-hard/inner-soft structure and having anexcessively large hardness distribution is hit with a driver, greatenergy loss occurs at this core. The energy loss results indeterioration of the resilience performance. When a core having anouter-hard/inner-soft structure and having an excessively large hardnessdistribution is hit with a short iron, a low spin rate is achieved. Thelow spin rate results in deterioration of the control performance.

An object of the present invention is to provide a golf ball that canattain a great flight distance upon hitting with a driver, and that isexcellent in a control performance achieved upon hitting with a shortiron.

SUMMARY OF THE INVENTION

The golf ball according to one aspect of the present invention has acore, a mid layer situated on the external side of the core, and a coversituated on the external side of the mid layer. The proportion of thevolume of the core relative to the volume of a phantom sphere of thegolf ball is no less than 76%. The difference (He−Ho) between the JIS-Chardness He of the surface of the core and the JIS-C hardness Ho of thecentral point of the core is 20 or greater and 40 or less. The JIS-Chardness Hc of the cover is less than the hardness Ho. At all points Pincluded in a zone away from the central point of the core at a distanceof 1 mm or greater and 15 mm or less, the following mathematicalexpression is satisfied.

H2−H1≦5

In the above mathematical expression, H1 represents the JIS-C hardnessat a point P1 that is located inside the point P along the radialdirection and away from the point P at a distance of 1 mm, and H2represents the JIS-C hardness at a point P2 that is located outside thepoint P along the radial direction and away from the point P at adistance of 1 mm.

In the golf ball according to the present invention, the core has anappropriate hardness distribution. This core is accompanied by lessenergy loss upon hitting with a driver. According to this golf ball, agreat flight distance is attained upon hitting with a driver. This golfball is excellent in a control performance achieved upon hitting with ashort iron.

Preferably, the JIS-C hardness Hc of the cover is no greater than 65.The cover has a thickness of preferably no greater than 0.8 mm.

Preferably, the JIS-C hardness Hm of the mid layer is no less than 90.The mid layer has a thickness of preferably no greater than 1.5 mm.

Preferably, a principal component of the base material of the cover is athermoplastic polyurethane. The polyol component of this thermoplasticpolyurethane is a polytetramethylene ether glycol having a numberaverage molecular weight of no greater than 1,500.

Preferably, the hardness Hm is greater than the hardness He. Preferably,the hardness He is 70 or greater and 95 or less. Preferably, thehardness Ho is 40 or greater and 80 or less. Preferably, the difference(Ho−Hc) between the hardness Ho and the hardness Hc is 3 or greater and15 or less.

The core may have a center and an envelope layer situated on theexternal side of the center. The center has a diameter of preferably 10mm or greater and 20 mm or less, and the envelope layer has a thicknessof preferably mm or greater and 18 mm or less. Preferably, the hardnessHe is greater than the surface hardness of the center. Preferably, thedifference (He−Hi) between the hardness He and the hardness Hi of theinnermost point of the envelope layer is 10 or greater and 25 or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partially cut off cross-sectional view illustrating agolf ball according to one embodiment of the present invention;

FIG. 2 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 1 of the present invention;

FIG. 3 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 2 of the present invention;

FIG. 4 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Example 3 of the present invention;

FIG. 5 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 1;

FIG. 6 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 2; and

FIG. 7 shows a graph illustrating a hardness distribution of the core ofa golf ball according to Comparative Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail accordingto the preferred embodiments with appropriate references to theaccompanying drawing.

A golf ball 2 shown in FIG. 1 has a spherical core 4, a mid layer 6situated on the external side of the core 4, and a cover 8 situated onthe external side of the mid layer 6. The core 4 has a spherical center10, and an envelope layer 12 situated on the external side of the center10. A large number of dimples 14 are formed on the surface of the cover8. Of the surface of the golf ball 2, a part other than the dimples 14is land 16. This golf ball 2 has a paint layer and a mark layer on theexternal side of the cover 8 although these layers are not shown in theFigure. The golf ball 2 may have a core which does not have an envelopelayer.

This golf ball 2 has a diameter of from 40 mm to 45 mm. From thestandpoint of conformity to a rule defined by the United States GolfAssociation (USGA), the diameter is preferably no less than 42.67 mm. Inlight of suppression of the air resistance, the diameter is preferablyno greater than 44 mm, and more preferably no greater than 42.80 mm. Theweight of this golf ball 2 is 40 g or greater and 50 g or less. In lightof attainment of great inertia, the weight is preferably no less than 44g, and more preferably no less than 45.00 g. From the standpoint ofconformity to a rule defined by the USGA, the weight is preferably nogreater than 45.93 g.

Preferably, the center 10 is obtained through crosslinking of a rubbercomposition. Illustrative examples of preferable base rubber includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light of theresilience performance, polybutadienes are preferred. When other rubberis used in combination with a polybutadiene, it is preferred that thepolybutadiene is included as a principal component. Specifically, thepercentage of the amount of the polybutadiene relative to the totalamount of the base rubber is preferably no less than 50% by weight, andmore preferably no less than 80% by weight. The percentage of cis-1,4bonds in the polybutadiene is preferably no less than 40%, and morepreferably no less than 80%.

The rubber composition for use in the center 10 contains aco-crosslinking agent. The co-crosslinking agent serves in achieving ahigh resilience of the center 10. Preferable examples of theco-crosslinking agent in light of the resilience performance includemonovalent or bivalent metal salts of an α,β-unsaturated carboxylic acidhaving 2 to 8 carbon atoms. Specific examples of the preferableco-crosslinking agent include zinc acrylate, magnesium acrylate, zincmethacrylate and magnesium methacrylate. In light of the resilienceperformance, zinc acrylate and zinc methacrylate are particularlypreferred.

In light of the resilience performance of the golf ball 2, the amount ofthe co-crosslinking agent is preferably no less than 5 parts by weight,and particularly preferably no less than 10 parts by weight relative to100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the co-crosslinking agent is preferably no greater than 30parts by weight, and more preferably no greater than 25 parts by weight,and particularly preferably no greater than 20 parts by weight relativeto 100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the center 10 includes anorganic peroxide together with the co-crosslinking agent. The organicperoxide serves as a crosslinking initiator. The organic peroxide isresponsible for the resilience performance of the golf ball 2. Examplesof suitable organic peroxide include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe organic peroxide is preferably no less than 0.1 part by weight, morepreferably no less than 0.2 part by weight, and particularly preferablyno less than 0.3 part by weight relative to 100 parts by weight of thebase rubber. In light of soft feel at impact, the amount of the organicperoxide is preferably no greater than 1.5 parts by weight, morepreferably no greater than 1.0 part by weight, and particularlypreferably no greater than 0.8 part by weight relative to 100 parts byweight of the base rubber.

Preferably, the rubber composition for use in the center 10 contains anorganic sulfur compound. Illustrative examples of preferable organicsulfur compound include mono-substituted forms such as diphenyldisulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide,bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide,bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide andbis(4-cyanophenyl)disulfide; di-substituted forms such asbis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide,bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfideand bis(2-cyano-5-bromophenyl)disulfide; tri-substituted forms such asbis(2,4,6-trichlorophenyl)disulfide andbis(2-cyano-4-chloro-6-bromophenyl)disulfide; tetra-substituted formssuch as bis(2,3,5,6-tetrachlorophenyl)disulfide; and penta-substitutedforms such as bis(2,3,4,5,6-pentachlorophenyl)disulfide andbis(2,3,4,5,6-pentabromophenyl)disulfide. The organic sulfur compound isresponsible for the resilience performance. Particularly preferredorganic sulfur compounds are diphenyl disulfide, andbis(pentabromophenyl)disulfide.

In light of the resilience performance of the golf ball 2, the amount ofthe organic sulfur compound is preferably no less than 0.1 part byweight, and more preferably no less than 0.2 part by weight relative to100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the organic sulfur compound is preferably no greater than1.5 parts by weight, more preferably no greater than 1.0 part by weight,and particularly preferably no greater than 0.8 part by weight relativeto 100 parts by weight of the base rubber.

Into the center 10 may be blended a filler for the purpose of adjustingthe specific gravity and the like. Illustrative examples of suitablefiller include zinc oxide, barium sulfate, calcium carbonate andmagnesium carbonate. The amount of the filler is determined ad libitumso that the intended specific gravity of the center 10 can beaccomplished. Particularly preferable filler is zinc oxide. Zinc oxideserves not only to adjust the specific gravity but also as acrosslinking activator.

An anti-aging agent, a coloring agent, a plasticizer, a dispersant,sulfur, a vulcanization accelerator and the like may be added to therubber composition for use in the center 10 as needed. In this rubbercomposition may be also dispersed crosslinked rubber powders orsynthetic resin powders.

In light of the resilience performance, the central hardness Ho of thecenter 10 is preferably no less than 40, more preferably no less than45, and particularly preferably no less than 50. In light of suppressionof the spin, the central hardness H1 is preferably no greater than 80,more preferably no greater than 75, and particularly preferably nogreater than 70. The central hardness Ho is measured by pushing a JIS-Ctype hardness scale on a central point of a section of a hemispherewhich had been obtained by cutting the center 10. For the measurement,an automated rubber hardness tester (“P1”, trade name, available fromKobunshi Keiki Co., Ltd.) equipped with this hardness scale is used.

The hardness of this center 10 gradually increases from the centralpoint toward the surface. The surface hardness of the center 10 isgreater than the central hardness Ho.

The center 10 has a diameter of preferably 10 mm or greater and 20 mm orless. By the center 10 having a diameter of no less than 10 mm,excellent feel at impact can be achieved. In this respect, the diameteris more preferably no less than 12 mm, and particularly preferably noless than 13 mm. The center 10 having a diameter of no greater than 20mm enables the envelope layer 12 having a sufficiently great thicknesscan be formed. In this respect, the diameter is more preferably nogreater than 18 mm, and particularly preferably no greater than 17 mm.

The envelope layer 12 is obtained through crosslinking of a rubbercomposition. Illustrative examples of preferable base rubber includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light of theresilience performance, polybutadienes are preferred. When other rubberis used in combination with a polybutadiene, it is preferred that thepolybutadiene is included as a principal component. Specifically, thepercentage of the amount of the polybutadiene relative to the totalamount of the base rubber is preferably no less than 50% by weight, andmore preferably no less than 80% by weight. The percentage of cis-1,4bonds in the polybutadiene is preferably no less than 40%, and morepreferably no less than 80%.

A co-crosslinking agent is preferably used in crosslinking the envelopelayer 12. Preferable examples of the co-crosslinking agent in light ofthe resilience performance include monovalent or bivalent metal salts ofan α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of the preferable co-crosslinking agent include zinc acrylate,magnesium acrylate, zinc methacrylate and magnesium methacrylate. Inlight of the resilience performance, zinc acrylate and zinc methacrylateare particularly preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe co-crosslinking agent is preferably no less than 20 parts by weight,and particularly preferably no less than 25 parts by weight relative to100 parts by weight of the base rubber. In light of soft feel at impact,the amount of the co-crosslinking agent is preferably no greater than 50parts by weight, more preferably no greater than 45 parts by weight, andparticularly preferably no greater than 35 parts by weight relative to100 parts by weight of the base rubber.

Preferably, the rubber composition for use in the envelope layer 12includes an organic peroxide together with the co-crosslinking agent.The organic peroxide serves as a crosslinking initiator. The organicperoxide is responsible for the resilience performance of the golf ball2. Examples of suitable organic peroxide include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of the resilience performance of the golf ball 2, the amount ofthe organic peroxide is preferably no less than 0.1 part by weight, morepreferably no less than 0.3 part by weight, and particularly preferablyno less than 0.5 part by weight relative to 100 parts by weight of thebase rubber. In light of soft feel at impact, the amount of the organicperoxide is preferably no greater than 2.0 parts by weight, morepreferably no greater than 1.5 parts by weight, and particularlypreferably no greater than 1.0 part by weight relative to 100 parts byweight of the base rubber.

Preferably, the rubber composition for use in the envelope layer 12contains an organic sulfur compound. The organic sulfur compounddescribed above in connection with the center 10 can be used for theenvelope layer 12. In light of the resilience performance of the golfball 2, the amount of the organic sulfur compound is preferably no lessthan 0.1 part by weight, and more preferably no less than 0.2 part byweight relative to 100 parts by weight of the base rubber. In light ofsoft feel at impact, the amount of the organic sulfur compound ispreferably no greater than 1.5 parts by weight, more preferably nogreater than 1.0 part by weight, and particularly preferably no greaterthan 0.8 part by weight relative to 100 parts by weight of the baserubber.

Into the envelope layer 12 may be blended a filler for the purpose ofadjusting the specific gravity and the like. Illustrative examples ofsuitable filler include zinc oxide, barium sulfate, calcium carbonateand magnesium carbonate. Powders constituted with a highly dense metalmay be also blended as the filler. Specific examples of the highly densemetal include tungsten and molybdenum. The amount of the filler isdetermined ad libitum so that the intended specific gravity of theenvelope layer 12 can be accomplished. Particularly preferable filler iszinc oxide. Zinc oxide serves not only to adjust the specific gravitybut also as a crosslinking activator. Various kinds of additives such assulfur, an anti-aging agent, a coloring agent, a plasticizer, adispersant and the like may be blended in an adequate amount in theenvelope layer as needed. Into the envelope layer 12 may be also blendedcrosslinked rubber powders or synthetic resin powders.

In the molding of the envelope layer 12, the center 10 is covered by twopieces of uncrosslinked or partially crosslinked half shell. The halfshells are compressed and heated. The heating causes a crosslinkingreaction, thereby completing an envelope layer 12. The crosslinkingtemperature is usually 140° C. or higher and 180° C. or lower. Thecrosslinking time period of the envelope layer 12 is usually 10 minutesor longer and 60 minutes or shorter.

In this envelope layer 12, the hardness gradually increases from theinnermost point toward the surface. In light of the resilienceperformance, the hardness He of the surface of the envelope layer 12(i.e., the surface of the core 4) is preferably no less than 70, andparticularly preferably no less than 75. In light of the feel at impact,the hardness He is preferably no greater than 95, more preferably nogreater than 90, and particularly preferably no greater than 87. Thehardness He is measured by pushing a JIS-C type hardness scale on thesurface of the core 4. For the measurement, an automated rubber hardnesstester “P1”, trade name, available from Kobunshi Keiki Co., Ltd.)equipped with this hardness scale is used.

In light of suppression of the spin, the difference (He−Hi) between thesurface hardness He of the envelope layer 12 and the hardness Hi of theinnermost point of the envelope layer 12 is preferably no less than 10,more preferably no less than 12, and particularly preferably no lessthan 15. In light of ease in manufacture and durability, the difference(He−Hi) is preferably no greater than 25.

The hardness Hi is measured on a hemisphere obtained by cutting the core4. By pushing a JIS-C type hardness scale on a section of thehemisphere, the hardness Hi is measured. The hardness scale is pushed ona region sandwiched between a first circle and a second circle. Thefirst circle corresponds to a boundary between the center and theenvelope layer 12. The second circle is concentric with the first circleand has a radius greater than the first circle by 1 mm. For themeasurement, an automated rubber hardness tester (“P1”, trade name,available from Kobunshi Keiki Co., Ltd.) equipped with this hardnessscale is used.

The envelope layer 12 has a thickness of preferably 8 mm or greater and18 mm or less. The envelope layer 12 having a thickness of no less than8 mm can suppress the spin. In this respect, the thickness is morepreferably no less than 9 mm, and particularly preferably no less than10 mm. The envelope layer 12 having a thickness of no greater than 18 mmenables the center 10 having a large diameter to be formed. The center10 having a large diameter can suppress the spin. In this respect, thethickness is more preferably no greater than 16 mm, and particularlypreferably no greater than 15 mm.

In light of suppression of the spin, the difference (He−Ho) between thesurface hardness He of the core 4 and the central hardness Ho of thecenter 10 is preferably no less than 20, and particularly preferably noless than 23. In light of ease of manufacture and the resilienceperformance of the core 4, the difference (He−Ho) is preferably nogreater than 40, and particularly preferably no greater than 35.

At all points P included in a zone away from the central point of thecore 4 at a distance of 1 mm or greater and 15 mm or less, the followingmathematical expression is satisfied.

H2−H1≦5

In this mathematical expression, H1 represents the JIS-C hardness of thepoint P1. The point P1 is located inside the point P along the radialdirection. The point P1 is away from the point P at a distance of 1 mm.In this mathematical expression, H2 represents the JIS-C hardness of thepoint P2. The point P2 is located outside the point P along the radialdirection. The point P2 is away from the point P at a distance of 1 mm.The hardness H1 and the hardness H2 are measured by pushing a JIS-C typehardness scale on a section of the hemisphere, which had been obtainedby cutting the center 10. For the measurement, an automated rubberhardness tester (“P1”, trade name, available from Kobunshi Keiki Co.,Ltd.) equipped with this hardness scale is used.

The proportion of the volume of the core 4 relative to the volume of thephantom sphere of the golf ball 2 is no less than 76%. In other words,this core 4 is large. This core 4 can serve in achieving a superiorresilience performance of the golf ball 2. This core 4 can suppress thespin of the golf ball 2. In these respects, this proportion is morepreferably no less than 79%, and particularly preferably no less than80%. The surface of the phantom sphere corresponds to the surface of thegolf ball 2 assumed as not having the dimples 14.

For the mid layer 6, a resin composition may be suitably used.Illustrative examples of the base polymer of this resin compositioninclude ionomer resins, styrene block-containing thermoplasticelastomers, thermoplastic polyester elastomers, thermoplastic polyamideelastomers and thermoplastic polyolefin elastomers.

Particularly preferable base polymer is an ionomer resin. The ionomerresins are highly elastic. As described later, this golf ball 2 has athin and soft cover 8. Therefore, upon hitting of this golf ball 2 witha driver, the mid layer 6 is greatly deformed. The mid layer containingthe ionomer resin is responsible for the resilience performance achievedupon shots with a driver. An ionomer resin and other resin may be usedin combination. When these are used in combination, the percentage ofthe amount of the ionomer resin relative to the total amount of the basepolymer is preferably no less than 50% by weight, more preferably noless than 70% by weight, and particularly preferably no less than 85% byweight, in light of the resilience performance.

Examples of preferred ionomer resin include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. Preferable binary copolymer comprises a 80% by weight ormore and 90% by weight or less α-olefin, and a 10% by weight or more and20% by weight or less α,β-unsaturated carboxylic acid. This binarycopolymer provides excellent resilience performance. Examples of otherionomer resin preferred include ternary copolymers formed with anα-olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms,and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms.Preferable ternary copolymer comprises a 70% by weight or more and 85%by weight or less α-olefin, a 5% by weight or more and 30% by weight orless α,β-unsaturated carboxylic acid, and a 1% by weight or more and 25%by weight or less α,β-unsaturated carboxylate ester. This ternarycopolymer provides excellent resilience performance. In the binarycopolymer and ternary copolymer, preferable α-olefin is ethylene andpropylene, and preferable α,β-unsaturated carboxylic acid is acrylicacid and methacrylic acid. Particularly preferred ionomer resin is acopolymer formed with ethylene, and acrylic acid or methacrylic acid.

In the binary copolymer and ternary copolymer, a part of the carboxylgroups may be neutralized with a metal ion. Illustrative examples of themetal ion for use in the neutralization include sodium ion, potassiumion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion andneodymium ion. The neutralization may be carried out with two or morekinds of the metal ions. Particularly suitable metal ion in light of theresilience performance and durability of the golf ball 2 is sodium ion,zinc ion, lithium ion and magnesium ion.

Specific examples of the ionomer resin include “Himilan® 1555”,“Himilan® 1557”, “Himilan® 1605”, “Himilan® 1706”, “Himilan® 1707”,“Himilan® 1856”, “Himilan® 1855”, “Himilan® AM7311”, “Himilan® AM7315”,“Himilan® AM7317”, “Himilan® AM7318”, “Himilan AM7329”, “Himilan®MK7320” and “Himilan® MK7329”, trade names, available from DuPont-MITSUI POLYCHEMICALS Co., Ltd.; “Surlyn® 6120”, “Surlyn® 6910”,“Surlyn® 7930”, “Surlyn® 7940”, “Surlyn® 8140”, “Surlyn® 8150”, “Surlyn®8940”, “Surlyn® 8945”, “Surlyn® 9120”, “Surlyn® 9150”, “Surlyn® 9910”,“Surlyn® 9945”, “Surlyn® AD8546”, “HPF 1000” and “HPF 2000”, tradenames, available from Du Pont Kabushiki Kaisha; and “IOTEK 7010”, “IOTEK7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, tradenames, available from EXXON Mobil Chemical Corporation.

Two or more kinds of the ionomer resins may be used in combination inthe mid layer 6. An ionomer resin neutralized with a monovalent metalion and an ionomer resin neutralized with a bivalent metal ion may beused in combination.

The mid layer 6 may also contain a highly elastic resin. Illustrativeexamples of the highly elastic resin include polybutylene terephthalate,polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamideimide, polyetherimide, polyether ether ketone, polyimide, polytetrafluoroethylene,polyaminobismaleimide, polybisamide triazole, polyphenyleneoxide,polyacetal, polycarbonate, acrylonitrile-butadiene-styrene copolymersand acrylonitrile-styrene copolymers.

Into the mid layer 6 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded. For forming the mid layer 6, a known procedure such as injectionmolding, compression molding and the like may be employed.

The mid layer 6 has the hardness Hm of preferably no less than 90. Themid layer 6 having the hardness Hm of no less than 90 can serve inachieving excellent resilience performance of the golf ball 2. Inaddition, with the mid layer 6 having a hardness Hm of no less than 90,an outer-hard/inner-soft structure of a sphere composed of the core 4and the mid layer 6 can be attained. The sphere having anouter-hard/inner-soft structure suppresses the spin of the golf ball 2.In these respects, the hardness Hm is particularly preferably no lessthan 92. In light of the feel at impact, the hardness Hm is preferablyno greater than 98, and particularly preferably no greater than 97. Inlight of suppression of the spin, it is preferred that the hardness Hmof the mid layer 6 is greater than the surface hardness He of the core4, and that the surface hardness He of the core 4 is greater than thesurface hardness of the center 10.

The hardness Hm is measured with a JIS-C type spring hardness scaleattached to an Auto Loading Durometer (automated rubber hardness tester,Kobunshi Keiki Co., Ltd., trade name “P1”). For the measurement, a slabformed by hot press is used. The slab has a thickness of about 2 mm. Theslab which had been stored at a temperature of 23° C. for two weeks isused for the measurement. When the measurement is carried out, threeslabs are overlaid. The slab constituted with the same resin compositionas that of the mid layer 6 is used for the measurement.

In light of suppression of the spin, the thickness of the mid layer 6 ispreferably no less than 0.3 mm, and particularly preferably no less than0.5 mm. In light of the feel at impact, the thickness is preferably nogreater than 1.5 mm, more preferably no greater than 1.2 mm, andparticularly preferably no greater than 1.0 mm.

The cover 8 is constituted with a resin composition. Illustrativeexamples of the base polymer of this resin composition includepolyurethanes, polyesters, polyamides, polyolefins, polystyrenes andionomer resins. In particular, a polyurethane is preferred. Apolyurethane is soft. When the golf ball 2 having a cover 8 in which apolyurethane is used is hit with a short iron, a great spin rate isattained. The cover 8 constituted with a polyurethane is responsible forthe control performance upon shots with a short iron. The polyurethaneis also responsible for the scuff resistance performance of the cover 8.

When this golf ball 2 is hit with a driver, long iron or middle iron,the sphere composed of the core 4 and the mid layer 6 is greatlydistorted due to a high head speed. Since this sphere has anouter-hard/inner-soft structure, the spin rate is suppressed. Due tosuppression of the spin rate, a great flight distance is attained. Whenthis golf ball 2 is hit with a short iron, less distortion of the sphereoccurs since the head speed is low. Behavior of the golf ball 2 uponhitting with a short iron predominantly varies depending on the cover 8.Since the cover 8 containing the polyurethane is soft, a great spin rateis attained. By the great spin rate, an excellent control performance isachieved. According to this golf ball 2, flight performances achievedupon shots with a driver, a long iron and a middle iron, and controlperformances achieved upon shots with a short iron are both achievedwith favorable balance.

When this golf ball 2 is hit, the cover 8 including a polyurethaneabsorbs impact. This absorption leads to a soft feel at impact achieved.In particular, when hit with a short iron or a putter, the cover 8 leadsto an excellent feel at impact achieved.

Into the cover 8, the polyurethane and other resin may be used incombination. When thus used in combination, the polyurethane is includedas a principal component of the base polymer in light of the spinperformance and the feel at impact. The percentage of the amount of thepolyurethane relative to the total amount of the base polymer ispreferably no less than 50% by weight, more preferably no less than 70%by weight, and particularly preferably no less than 85% by weight.

A thermoplastic polyurethane and a thermosetting polyurethane may beused in the cover 8. In light of the productivity, a thermoplasticpolyurethane is preferred. The thermoplastic polyurethane includes apolyurethane component as a hard segment, and a polyester component or apolyether component as a soft segment.

The polyurethane contains a polyol component. As the polyol, a polymerpolyol is preferred. Specific examples of the polymer polyol include:polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropyleneglycol (PPG) and polytetramethylene ether glycol (PTMG); condensedpolyester polyols such as polyethylene adipate (PEA), polybutyleneadipate (PBA) and polyhexamethylene adipate (PHMA); lactone basedpolyester polyols such as poly-ε-caprolactone (PCL); polycarbonatepolyols such as polyhexamethylene carbonate; and acrylic polyols. Two ormore kinds of the polyol may be used in combination.

Particularly, a polytetramethylene ether glycol is preferred. A spinrate attained upon hitting of the golf ball 2 with a short iron has agreat correlation with the content of the polytetramethylene etherglycol. On the other hand, a spin rate attained upon hitting of the golfball 2 with a driver has a less correlation with the content of thepolytetramethylene ether glycol. The golf ball 2 in which thepolyurethane contains an appropriate amount of a polytetramethyleneether glycol is excellent in both terms of the flight performanceachieved upon hitting with a driver, and the control performanceachieved upon hitting with a short iron.

In light of the control performance, the polyol has a number averagemolecular weight of preferably no less than 200, more preferably no lessthan 400, and particularly preferably no less than 650. In light ofsuppression of the spin, the molecular weight is preferably no greaterthan 1,500, more preferably no greater than 1,200, and particularlypreferably no greater than 850.

The number average molecular weight is measured with a gel permeationchromatography. The measurement conditions are as in the following.

Apparatus: HLC-8120GPC (Tosoh Corporation)

Eluent: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSKgel Super HM-M (Tosoh Corporation)

Amount of sample: 5 microliter

Flow rate: 0.5 ml/min

Standard substance: polystyrene (Tosoh Corporation, “PStQuick Kit-H”)

Examples of the isocyanate component in the polyurethane include:aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluenediisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI) and paraphenylene diisocyanate (PPDI); alicyclic polyisocyanatessuch as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenatedxylylene diisocyanate (H₆XDI) and isophorone diisocyanate (IPDI); andaliphatic polyisocyanates such as hexamethylene diisocyanate (HDI). Twoor more polyisocyanates may be used in combination. In light of theweather resistance, TMXDI, XDI, HDI, H₆XDI, IPD₁ and H₁₂MDI arepreferred.

The polyurethane may contain a chain extender as a component thereof.Illustrative examples of the chain extender include low molecular weightpolyols and low molecular weight polyamines.

The low molecular weight polyols are exemplified by diols, triols,tetraols and hexaols. Specific examples of the diol include ethyleneglycol, diethylene glycol, propane diol, dipropylene glycol, butanediol,neopentyl glycol, pentanediol, hexanediol, heptanediol and octanediol.Specific examples of the triol include glycerin, trimethylolpropane andhexanetriol. Specific examples of the tetraol include pentaerythritoland sorbitol.

The low molecular weight polyamines are exemplified by aliphaticpolyamines, monocyclic aromatic polyamines and polycyclic aromaticpolyamines. Specific examples of the aliphatic polyamine includeethylene diamine, propylene diamine, butylene diamine and hexamethylenediamine. Specific examples of the monocyclic aromatic polyamine includephenylene diamine, toluene diamine, dimethyltoluene diamine,dimethylthiotoluene diamine and xylylene diamine.

The cover 8 may be molded from a composition containing a thermoplasticpolyurethane and an isocyanate compound. During or following molding ofthe cover 8, the polyurethane is crosslinked by this isocyanatecompound.

Into the cover 8 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded.

The JIS-C hardness Hc of the cover 8 is no greater than 65. By employingsuch a soft cover 8, a favorable control performance upon shots with ashort iron can be achieved. In light of the control performance, thehardness Hc is more preferably no greater than 60, still more preferablyno greater than 55, and particularly preferably no greater than 50. Whenthe hardness is too low, the flight performance achieved upon shots witha driver may be insufficient. In this respect, the hardness ispreferably no less than 20, more preferably no less than 25, andparticularly preferably no less than 35. For the measurement of thehardness Hc, a slab constituted with the same resin composition as theresin composition of the cover 8 is used. The measuring method issimilar to the measuring method of the hardness Hm of the mid layer 6.

The hardness Hc of the cover 8 is less than the central hardness Ho ofthe core 4. This golf ball 2 is excellent in the control performanceachieved upon shots with a short iron. In light of the controlperformance, the difference (Ho−Hc) is preferably no less than 3, morepreferably no less than 5, and particularly preferably no less than 8.The difference (Ho−Hc) is preferably no greater than 15.

In light of the flight performance achieved upon shots with a driver,the cover 8 has a thickness of preferably no greater than 0.8 mm, morepreferably no greater than 0.6 mm, still more preferably no greater than0.5 mm, and particularly preferably no greater than 0.4 mm. In light ofthe control performance achieved upon shots with a short iron, thethickness is preferably no less than 0.10 mm, and particularlypreferably no less than 0.15 mm.

For forming the cover 8, a known procedure may be employed such asinjection molding, compression molding or the like. Dimples 14 areformed by way of pimples formed on the cavity face of the mold when thecover 8 is molded.

In light of the feel at impact, the amount of compressive deformation Dbof the golf ball 2 is preferably no less than 2.3 mm, more preferably noless than 2.4 mm, and particularly preferably no less than 2.5 mm. Inlight of the resilience performance, the deformation Db is preferably nogreater than 3.5 mm, more preferably no greater than 3.2 mm, andparticularly preferably no greater than 3.0 mm.

Upon measurement of the deformation Db, the golf ball 2 is placed on ahard plate made of metal. A cylinder made of metal gradually descendstoward this golf ball 2. The golf ball 2 interposed between the bottomface of the cylinder and the hard plate is deformed. A migrationdistance of the cylinder, starting from the state in which an initialload of 98 N is applied to the golf ball 2 up to the state in which afinal load of 1,274 N is applied thereto is measured.

The golf ball 2 may have a reinforcing layer between the mid layer 6 andthe cover 8. The reinforcing layer firmly adheres to the mid layer 6,and firmly adheres also to the cover 8. Due to the reinforcing layer,detachment of the cover 8 from the mid layer 6 can be suppressed. Asdescribed above, this golf ball 2 has thin cover 8. When a golf ballwhich has a thin cover is hit with an edge of a clubface, a wrinkle isliable to be generated. The reinforcing layer suppresses generation ofsuch a wrinkle.

For the base polymer of the reinforcing layer, a two-component curedthermosetting resin may be suitably used. Specific examples of thetwo-component cured thermosetting resin include epoxy resins, urethaneresins, acrylic resins, polyester based resins and cellulose basedresins. In light of the strength and durability of the reinforcinglayer, two-component cured epoxy resins and two-component cured urethaneresins are preferred.

The reinforcing layer may include additives such as a coloring agent(typically, titanium dioxide), a phosphate based stabilizer, anantioxidant, a light stabilizer, a fluorescent brightening agent, anultraviolet absorbent, a blocking preventive agent and the like. Theadditive may be added either to the base material of the two-componentcured thermosetting resin, or to the curing agent of the two-componentcured thermosetting resin.

The reinforcing layer is obtained by coating a liquid, which had beenprepared by dissolving or dispersing a base material and a curing agentin a solvent, on the surface of the mid layer 6. In light of theworkability, coating with a spray gun is preferred. The solvent isvolatilized after the coating to permit a reaction of the base materialwith the curing agent, thereby forming the reinforcing layer.

In light of suppression of the wrinkle, the reinforcing layer has athickness of preferably no less than 3 μm, and more preferably no lessthan 5 μm. In light of ease of forming the reinforcement layer, thethickness is preferably no greater than 300 μm, more preferably nogreater than 50 μm, and particularly preferably no greater than 20 μm.The thickness is measured by observation of the cross section of thegolf ball 2 with a microscope. When the surface of the mid layer 6 hasroughness resulting from a surface roughening treatment, the thicknessis measured immediately above the protruded portion.

In light of suppression of the wrinkle, the reinforcing layer has apencil hardness of preferably no less than 4 B, and more preferably noless than B. In light of less loss of the force during transfer from thecover 8 to the mid layer 6 upon hitting of the golf ball 2, thereinforcing layer has a pencil hardness of preferably no greater than 3H. The pencil hardness is measured in accordance with a standard of “JISK5400”.

EXAMPLES Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (“BR-730”, trade name, available from JSRCorporation), 20 parts by weight of zinc diacrylate, 5 parts by weightof zinc oxide, an adequate amount of barium sulfate, 0.5 part by weightof diphenyl disulfide and 0.7 part by weight of dicumyl peroxide. Thisrubber composition (1) was placed into a mold having upper and lowermold half each having a hemispherical cavity, and heated at atemperature of 170° C. for 15 minutes to obtain a center having adiameter of 15 mm.

A rubber composition (4) was obtained by kneading 100 parts by weight ofa high-cis polybutadiene (“BR-730”, supra), 35 parts by weight of zincdiacrylate, 5 parts by weight of zinc oxide, an adequate amount ofbarium sulfate, 0.5 part by weight of diphenyl disulfide and 0.7 part byweight of dicumyl peroxide. Half shells were formed from this rubbercomposition (4). The aforementioned center was covered by two pieces ofthe half shell. The center and the half shells were placed into a moldhaving upper and lower mold half each having a hemispherical cavity, andheated at a temperature of 170° C. for 20 min to obtain a core having adiameter of 39.7 mm. An envelope layer was formed from the rubbercomposition (4). The amount of barium sulfate was adjusted such that theenvelope layer has a specific gravity identical to the specific gravityof the center, and the ball has a weight of 45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight ofan ionomer resin (“Surlyn® 8945”, supra), and 50 parts by weight ofother ionomer resin (“Himilan® AM7329”, supra) in a biaxial kneadingextruder. The core was placed into a mold having upper and lower moldhalf each having a hemispherical cavity. The resin composition (a) wasinjected around the core by injection molding, whereby a mid layer wasformed. This mid layer had a thickness of 1.0 mm.

A paint composition containing a two-component cured epoxy resin as abase polymer (“POLIN 750LE”, trade name, available from Shinto PaintCo., Ltd.) was prepared. The base material liquid of this paintcomposition consists of 30 parts by weight of a bisphenol A type solidepoxy resin and 70 parts by weight of a solvent. The curing agent liquidof this paint composition consists of 40 parts by weight of denaturedpolyamide amine, 55 parts by weight of a solvent and 5 parts by weightof titanium dioxide. The weight ratio of the base material liquid andthe curing agent liquid was 1/1. This paint composition was coated onthe surface of the mid layer with a spray gun, and kept in an atmosphereof 40° C. for 24 hrs to give a reinforcing layer. This reinforcing layerhad a thickness of 10 μm.

A resin composition (b) was obtained by kneading 100 parts by weight ofa thermoplastic polyurethane elastomer (“Elastollan® XNY85A”, tradename, available from BASF Japan Ltd.) and 4 parts by weight of titaniumdioxide in a biaxial kneading extruder. Half shells were obtained fromthis resin composition (b) with compression molding. A sphere composedof the core, the mid layer and the reinforcing layer was covered by twopieces of the half shell. The sphere and half shells were placed into afinal mold having upper and lower mold half each having a hemisphericalcavity and being provided with a large number of pimples on the cavityface thereof. A cover was obtained by compression molding. This coverhad a thickness of 0.5 mm. Dimples having a shape inverted from theshape of the pimple were formed on the cover. A clear paint including atwo-component cured polyurethane as a base material was applied on thiscover to give a golf ball of Example 1 having a diameter of 42.7 mm.

Examples 2 to 7 and Comparative Examples 1 to 4

Golf balls of Examples 2 to 7, and Comparative Examples 1 to 4 wereobtained in a similar manner to Example 1 except that specifications ofthe center, the envelope layer, the mid layer and the cover were aslisted in Tables 3 to 5 below. Details of the rubber compositions of thecore are presented in Table 1 below. Details of the resin compositionsof the mid layer and the cover are presented in Table 2 below. The golfball according to Comparative Example 1 does not have an envelope layer.

[Shot with Driver (W#1)]

A driver with a titanium head (SRI Sports Limited, trade name “SRIXONW505”, shaft hardness: X, loft angle:) 8.5° was attached to a swingmachine available from Golf Laboratory Co. Then the golf ball was hitunder a condition to give the head speed of 50 m/sec. The ball speed andspin rate immediately after the hitting, and the distance from thelaunching point to the point where the ball stopped were measured. Meanvalues of the data obtained by measuring 12 times are shown in Tables 3to 5 below.

[Shot with Short Iron]

A sand wedge (SW) was attached to a swing machine available from GolfLaboratory Co. Then the golf ball was hit under a condition to give thehead speed of 21 m/sec, and the spin rate immediately after the hittingwas measured. Mean values of the data obtained by measuring 12 times areshown in Tables 3 to 5 below.

[Feel at Impact]

The golf balls were hit by ten golf players with a sand wedge, and aninterview was conducted on the feel at impact. Based on the number ofgolf players who evaluated that “the feel at impact was favorable”,rating was performed according to the following criteria.

A: 8 or more

B: 6-7

C: 4-5

D: 3 or fewer

The results are shown in the following Tables 3 to 5.

TABLE 1 Composition of Core (part by weight) (1) (2) (3) (4) (5) BR-730100 100 100 100 100 Zinc diacrylate 20 32 34 35 37 Zinc oxide 5 5 5 5 5Barium sulfate * * * * * Diphenyl 0.5 0.5 0.5 0.5 0.5 disulfide Dicumylperoxide 0.7 0.7 0.7 0.7 0.7 * Adequate amount

TABLE 2 Composition of Mid Layer and Cover (part by weight) (a) (b) (c)(d) (e) (f) (g) (h) Surlyn ® 8945 50 — — — — — — — Himilan ® 50 — — — —— — — AM7329 Polyurethane *1 — 100  — — — — — — Polyurethane *2 — — 100 — — — — — Polyurethane *3 — — — 100  — — — — Polyurethane *4 — — — —100  — — — Polyurethane *5 — — — — — 100  — — Polyurethane *6 — — — — —— 100  — Polyurethane *7 — — — — — — — 100  Titanium dioxide —  4  4  4 4  4  4  4 Hardness (JIS-C) 94 47 56 67 45 42 42 38 Hardness 64 32 3847 30 28 28 25 (Shore D)The details of the polyurethane shown in Table 2 are as in thefollowing.

Polyurethane *1

“Elastollan® XNY85A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol

Component: 1800

polyurethane *2

“Elastollan® XNY90A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 1800

polyurethane *3

“Elastollan® XNY97A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 1800

Polyurethane *4

“Elastollan® XNY83A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 1500

Polyurethane *5

“Elastollan® XNY80A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 1000

Polyurethane *6

“Elastollan® XNY80A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 850

Polyurethane *7

“Elastollan® XNY75A”, trade names, available from BASF Japan Ltd.

Polyol component: polytetramethylene ether glycol

Number average molecular weight of the polyol component: 650

TABLE 3 Evaluation Results Example 1 Example 2 Example 3 Example 4Center Composition  (1)  (1)  (1)  (1) Crosslinking temperature 170° C.170° C. 170° C. 170° C. time (min) 15 15 15 15 Diameter (mm) 15 18 15 15Envelope Composition  (4)  (4)  (4)  (4) layer Crosslinking temperature170° C. 170° C. 170° C. 170° C. time (min) 20 20 20 20 Core Diameter(mm)   39.7   40.1   40.3   39.7 Volume proportion (%)   80.4   82.8  84.1   80.4 Hardness Ho (JIS-C) 60 60 60 60 Hardness He (JIS-C) 85 8585 85 He − Ho 25 25 25 25 Hardness distribution FIG. 2 FIG. 3 FIG. 4FIG. 2 H2-H1 (maximum value)  4  4  4  4 Mid Composition (a) (a) (a) (a)layer Hardness (JIS-C) 94 94 94 94 Thickness (mm)   1.0   1.0   0.9  1.0 Cover Composition (b) (c) (b) (e) Hardness (JIS-C) 47 56 47 45Thickness (mm)   0.5   0.3   0.3   0.5 Ball Deformation Db (mm)    2.80   2.85    2.85    2.80 W #1 Ball speed (m/s)   73.3   73.4   73.5  73.3 Spin (rpm) 2380  2290  2390  2320  Flight distance (m)  246.5 248.5  247.5  247.5 SW Spin (rpm) 6700  6530  6670  6710  Feel atimpact A B A A

TABLE 4 Evaluation Results Example 5 Example 6 Example 7 CenterComposition (1) (1) (1) Cross- temperature 170° C. 170° C. 170° C.linking time (min) 15 15 15 Diameter (mm) 15 15 15 Envelope Composition(4) (4) (4) layer Cross- temperature 170° C. 170° C. 170° C. linkingtime (min) 20 20 20 Core Diameter (mm) 39.7 39.7 39.7 Volume proportion(%) 80.4 80.4 80.4 Hardness Ho (JIS-C) 60 60 60 Hardness He (JIS-C) 8585 85 He-Ho 25 25 25 Hardness distribution FIG. 2 FIG. 2 FIG. 2 H2-H1(maximum value) 4 4 4 Mid Composition (a) (a) (a) layer Hardness (JIS-C)94 94 94 Thickness (mm) 1.0 1.0 1.0 Cover Composition (f) (g) (h)Hardness (JIS-C) 42 42 38 Thickness (mm) 0.5 0.5 0.5 Ball Deformation Db(mm) 2.80 2.80 2.80 W #1 Ball speed (m/s) 73.3 73.3 73.3 Speed (rpm)2300 2220 2330 Flight destance (m) 248.0 248.5 247.5 SW Spin (rpm) 66906610 6750 Feel at impact A A A

TABLE 5 Evaluation Results Compa. Compa. Compa. Compa. Example 1 Example2 Example 3 Example 4 Center Composition  (5)  (1)  (1)  (1)Crosslinking temperature 170° C. 170° C. 170° C. 170° C. 20 15 15 15Diameter (mm)   39.7 25 15 15 Envelope Composition —  (5)  (2)  (4)layer Crossliking temperature — 170° C. 170° C. 170° C. time (min) — 2020 20 Core Diameter (mm)   39.7   39.1   38.5   39.7 Volume proportion(%)   80.4   76.8   73.3   80.4 Hardness Ho (JIS-C) 66 60 60 60 HardnessHe (JIS-C) 84 87 82 85 He − Ho 18 27 22 25 Hardness distribution FIG. 5FIG. 6 FIG. 7 FIG. 2 H2-H1 (maximum value)   2.4  6  1  4 MidComposition (a) (a) (a) (a) layer Hardness (JIS-C) 94 94 94 94 Thickness(mm)   1.0   1.0   1.6   1.0 Cover Composition (b) (b) (b) (d) Hardness(JIS-C) 47 47 47 67 Thickness (mm)   0.5   0.8   0.5   0.5 BallDeformation Db (mm)    2.80    2.80    2.80    2.75 W #1 Ball speed(m/s)   73.3   73.0   73.1   73.3 Spin (rpm) 2480  2400  2400  2230 Flight distance (m)  245.0  244.5  245.5  249.5 SW Spin (rpm) 6720 6670  6610  6330  Feel at impact A A B C

As is shown in Tables 3 to 5, the golf balls according to Examples areexcellent in various performances. Therefore, advantages of the presentinvention are clearly suggested by these results of evaluation.

The golf ball according to the present invention can be used for theplay at the golf course, and the practice at the driving range. Theforegoing description is just for illustrative examples; therefore,various modifications can be made in the scope without departing fromthe principles of the present invention.

1. A golf ball comprising a core, a mid layer situated on the externalside of the core, and a cover situated on the external side of the midlayer, wherein: the proportion of the volume of the core relative to thevolume of the phantom sphere of the golf ball is no less than 76%; thedifference (He−Ho) between the JIS-C hardness He of the surface of thecore and the JIS-C hardness Ho of the central point of the core is 20 orgreater and 40 or less; the JIS-C hardness Hc of the cover is less thanthe hardness Ho; and at all points P included in a zone away from thecentral point of the core at a distance of 1 mm or greater and 15 mm orless, the following mathematical expression is satisfied:H2−H1≦5 in the above mathematical expression, H1 represents the JIS-Chardness at a point P1 that is located inside the point P along theradial direction and away from the point P at a distance of 1 mm, and H2represents the JIS-C hardness at a point P2 that is located outside thepoint P along the radial direction and away from the point P at adistance of 1 mm.
 2. The golf ball according to claim 1, wherein theJIS-C hardness Hc of the cover is no greater than
 65. 3. The golf ballaccording to claim 1, wherein the cover has a thickness of no greaterthan 0.8 mm.
 4. The golf ball according to claim 1, wherein: a principalcomponent of the base material of the cover is a thermoplasticpolyurethane; and a polyol component of the thermoplastic polyurethaneis a polytetramethylene ether glycol having a number average molecularweight of no greater than 1,500.
 5. The golf ball according to claim 1,wherein the JIS-C hardness Hm of the mid layer is no less than
 90. 6.The golf ball according to claim 1, wherein the mid layer has athickness of no greater than 1.5 mm.
 7. The golf ball according to claim1, wherein the JIS-C hardness Hm of the mid layer is greater than thehardness He.
 8. The golf ball according to claim 1, wherein the hardnessHe is 70 or greater and 95 or less.
 9. The golf ball according to claim1, wherein the hardness Ho is 40 or greater and 80 or less.
 10. The golfball according to claim 1, wherein the difference (Ho−Hc) between thehardness Ho and the hardness Hc is 3 or greater and 15 or less.
 11. Thegolf ball according to claim 1, wherein the core has a center, and anenvelope layer situated on the external side of the center.
 12. The golfball according to claim 11, wherein: the center has a diameter of 10 mmor greater and 20 mm or less; and the envelope layer has a thickness of8 mm or greater and 18 mm or less.
 13. The golf ball according to claim11, wherein the hardness He is greater than the surface hardness of thecenter.
 14. The golf ball according to claim 11, wherein the difference(He−Hi) between the hardness He and the hardness Hi of the innermostpoint of the envelope layer is 10 or greater and 25 or less.